This invention relates to spark ignition type internal combuation engines in general, and more particularly it is concerned with a two-cycle internal combustion engine of the type described capable of developing high output power and operating at high speed.
Heretofore, it has been usual practice to design two stroke cycle spark ignition type internal combustion engines of light weight capable of developing high output power with an eye on increasing output power while reducing a fuel consumption rate, particularly in a high-speed rotation range (over about 6000 rpm), at the cost of economy on fuel consumption in low- and medium-speed rotation ranges. However, there has in recent years been a demand for an improved fuel consumption rate in all the rotation ranges in this type of engine, and it is known that proposals have been made to use an ignition device as a means for solving the problem which optimizes ignition timing in all the rotation ranges. However, a two-cycle spark ignition type internal combustion engine capable of developing high output power and operating at high speed has a specific ignition characteristic. More specifically, when it is desired to prevent abnormal combustion that might occur due to an inordinate delay in ignition at high speed rotation, the desired ignition timing characteristic for this type of engine is such that the ignition advancer is delayed in a high-speed rotation range as compared with low- and medium-speed rotation range. Particularly, if the air-fuel ratio of a fuel-air mixture is made nearer to the theoretical air-fuel ratio rather than being enriched for improving fuel consumption in the high-speed rotation range, then the rate of occurrence of abnormal combustion increases sharply, and one is forced to further delay the ignition timing to avoid damage to the engine due to the abnormal combustion. Thus the ignition timing for the high-speed rotation range is markedly delayed in the high-speed rotation range than in the low- and medium-speed rotation ranges.
However, it is difficult for an ordinary ignition device (an ordinary contactless ignition device, for example) of low cost to satisfy the aforesaid advance requirements over the entire rotation range, and in actual practice there is the tendency that the output power and the fuel consumption rate in the low- and medium speed rotation ranges are sacrificed because the ignition timing for the high-speed rotation range is important for improving the fuel consumption rate and preventing abnormal combustion.
Meanwhile rapid combustion is effective in improving thermal efficiency in a spark ignition type internal combustion engine. However, in a two-cycle engine capable of developing high output power and operating at high speed, rapid combustion at high speed operation and at high load causes a marked rise in the temperature of combustion gas in the cylinders, so that what is referred to as a knock phenomenon may occur and cause ignition to take place by the spreading flames prior to initiation of ignition. When this phenomenon happens, a large quantity of heat is transmitted to the walls of the combustion chamber and the piston over the above the ability thereof to dissipate heat, resulting in the development of seizure.
A proposal has been made to use means for reducing the volume of the exhaust per cylinder and increasing the number of the cylinders to reduce the distance that should be covered by the flames when they spread, in order to effect rapid combustion and yet to prevent abnormal combustion. However, such means cannot be used in other engines than stationary engines because of an increase in the weight of the engine. Another means available relies on the provision of a plurality of ignition plugs for each cylinder to reduce the distance that should be covered by the flames when they spread. However, such means would suffer the aforesaid disadvantage of being unable to satisfactorily prevent abnormal combustion in the high-speed rotation range because of a reduction in the delay in ignition and the rapid progress of combustion, although improvements might be provided to the output power and the fuel consumption rate in the low- and medium-speed rotation ranges. If an enriched mixture is used with the aforesaid means to avoid abnormal combustion, then the fuel consumption rate in the high-speed rotation range would worsen, and it would become necessary to use a special fuel supply means for supplying an enriched mixture to the engine only in the high-speed rotation range, because the use of an ordinary cheap fuel supply means would result in an enriched mixture being fed to the engine even in the low- and medium-speed rotation ranges.